Intel probably has patents covering the code in the FSP, so that it could destroy Purism if it wanted to. A patent dispute in court costs about half a million dollars in legal fees.
For reverse engineering, you have one team that studies the product and writes a detailed description of what it does. Then another team which never looks at the original product, then takes that description and re-engineer it. If the FSP contains some kind of encryption or code obfuscation to hide its contents, Intel could also file a complaint with the US government that the first team is violating section 1201 of the Digital Millennium Copyright Act (DMCA) that says it is “unlawful to circumvent technological measures used to prevent unauthorized access to copyrighted works.” I have no idea how a court would view that, but making a successful argument in court would probably be expensive.
However, Intel won’t do any of these things, because attacking one of its customers who is buying its chips is simply bad business. Intel doesn’t care about Purism’s tiny number of orders, but it does care a lot about its Linux server business, which is why Intel is the largest contributor to the Linux kernel. If Intel attacked Purism, many of the companies that build Linux servers would switch to AMD and Intel would lose billions in revenue. All the work that Intel has done for the last 2 decades to establish itself as a Linux-friendly company would be flushed down the drain, if it attacked Purism, and it won’t want to hand that business to AMD.
Intel might have licensed some of the tech in the FSP, so it legally can’t share it, or it might not want its competitors to know how its CPU operates, but I can’t see Intel attacking Purism, when Intel is trying to keep the Linux server companies from switching to AMD and ARM CPUs, and trying to convince laptop makers to not switch to Snapdragon.